Inverters have been employed in many types of electrical equipments, as an efficient variable-speed control unit. Inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, followed by applying a voltage twice as high as the inverter output voltage at the maximum. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT, is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipments, such as inverter-related equipments, for example, high-speed switching devices, inverter motors, and transformers, insulated wires made of enameled wires are mainly used as magnet wires in the coils. Further, as described above, since a voltage almost twice as high as the inverter output voltage is applied in inverter-related equipments, it has become required to minimize the inverter surge deterioration of the enameled wire, which is one of the materials constituting the coils of those electrical equipments.
In the meantime, partial discharge deterioration is a complicated phenomenon in which an electrical-insulation material undergoes, for example, molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge of the insulating material, sputtering deterioration, thermal fusion or thermal decomposition deterioration caused by local temperature rise, and chemical deterioration caused by ozone generated due to discharge. For this reason, reduction in thickness, for example, is observed in the electrical-insulation materials, which have been deteriorated as a result of actual partial discharge.
It has been believed that inverter surge deterioration of an insulated wire also proceeds by the same mechanism as in the case of general partial discharge deterioration. Namely, inverter surge deterioration of an enameled wire is a phenomenon in which partial discharge occurs in the insulated wire due to the surge voltage with a high peak value, which is occurred at the inverter, and the coating of the insulated wire causes partial discharge deterioration as a result of the partial discharge; in other words, the inverter surge deterioration of an insulated wire is high-frequency partial discharge deterioration.
In order to prevent the inverter surge deterioration, insulated wires that are able to withstand several hundred volts of surge voltage have been required for the recent electrical equipment. That is, there is a demand for insulated wires that have a partial discharge inception voltage of 500 V or more. Herein, the partial discharge inception voltage is a value that is measured by a commercially available apparatus called partial discharge tester. Measurement temperature, frequency of the alternating current voltage to be used, measurement sensitivity, and the like are values that may vary as necessary, but the above-mentioned value is an effective value of the voltage at which partial discharge occurs, which is measured at 25° C., 50 Hz, and 10 pC.
When the partial discharge inception voltage is measured, a method is used in which the most severe condition possible in the case where the insulated wire is used as a magnet wire is envisaged, and a specimen shape is formed which can be observed in between two closely contacting insulated wires. For example, in the case of an insulated wire having a circular cross-section, two insulated wires are brought into linear contact by spiral twisting the wires together, and a voltage is applied between the two insulated wires. Alternatively, in the case of an insulated wire having a rectangular cross-section, use is made of a method of bringing two insulated wires into planar contact through the planes, which are the long sides of the insulated wires, and applying a voltage between the two insulated wires.
In order to obtain an insulated wire that does not cause partial discharge, that is, having a high partial discharge inception voltage, so as to prevent the deterioration of the enamel layer of the insulated wire caused by the partial discharge, it is thought to utilize a method of using a resin having a low dielectric constant in the enamel layer or increasing the thickness of the enamel layer. However, the resins of commonly used resin varnishes generally have a dielectric constant between 3 and 5, and none of the resins have particular low dielectric constant. Further, upon considering other properties (heat resistance, solvent resistance, flexibility, and the like) required from the enamel layer, it is not necessarily possible to select actually a resin having a low dielectric constant. Therefore, in order to obtain a high partial discharge inception voltage, it is indispensable to increase the thickness of the enamel layer. When the resins having a dielectric constant of 3 to 5 are used in the enamel layer, if it is intended to obtain a targeted partial discharge inception voltage of 500 V or higher, it is necessary based on the experience to set the thickness of the enamel layer at 60 tem or more.
However, to thicken the enamel layer, the number of times for passing through a baking furnace increases in a production process thereof, whereby making a film composed of copper oxide on a copper conductor surface thicker, this in turn, causing lowering in adhesion between the conductor and the backed enamel layer. For example, in the case of obtaining an enamel layer with thickness 60 μm or more, the number of passages through the baking furnace exceeds 12 times. It has been known that if this number of passages exceeds 12 times, the adhesive force between the conductor and the enamel layer is conspicuously lowered.
It is also thought to utilize a method of increasing the thickness that can be formed by a single baking step, in order not to increase the number of passages through the baking furnace. However, this method has a drawback that the solvent of the varnish is not completely vaporized and remains in the enamel layer as voids.
In the meantime, conventionally, attempts to enhance properties (properties other than the partial discharge inception voltage) by providing a coated resin at the outer side of the enamel wire were made. For example, Patent Literatures 1 and 2 are cited as a conventional art of providing an extrusion-coated layer on an enamel layer. In the insulated wire that has been provided with the coated resin, adhesiveness between the enamel layer and the coated resin is also required. However, the techniques disclosed in Patent Literatures 1 and 2 were not necessarily satisfactory for the thickness of the enamel layer or the extrusion-coated layer or the like, from the standpoint of balancing between the partial discharge inception voltage and the adhesiveness between the conductor and the enamel layer.
On the other hand, Patent Literature 3 is cited as a conventional art of addressing problems stemming from the partial discharge inception voltage and the adhesiveness between the conductor and the enamel layer.
Further, it has become demanded to further improve various performances, such as heat resistance, mechanical properties, chemical properties, electrical properties, and reliability, in the electrical equipments developed in recent years, as compared to the conventional electrical equipments. Under the situations, excellent abrasion resistance, thermal aging resistance property, and solvent resistance have become required from insulated wires, such as enameled wires, that are used as magnet wires for electrical equipments for aerospace use, electrical equipments for aircraft, electrical equipments for nuclear power, electrical equipments for energy, and electrical equipments for automobiles. For example, in the recent years, for electrical equipments, it sometimes has been required to show an excellent thermal aging resistance over a long period of time of use.
On the other hand, recently, advance of the electrical equipment represented by motors or transformers, has been progressed resulting in size reduction and improved performance, and thus it becomes usual in many cases that insulated wires are used in such a way that they are pushed into a quite small space to pack. Specifically, it is no exaggeration to say that the performance of a rotator, such as a motor, is determined by how many electrical wires can be held in a stator slot. As a result, the ratio of the sectional area of conductors to the sectional area of the stator slot (space factor) has been required to be particularly highly increased in recent years.
For example, when electrical wires each having a circular cross-section are closely packed at the inside of a stator slot, the space serving as dead space and the cross-sectional area of the respective insulation coating become important factors. For this reason, users attempt to increase the packing factor as much as possible, by press-fitting more electrical wires into a stator slot, up to a extent that the electrical wire having a circular cross-section causes deformation. However, since reducing the cross-sectional area of the insulation coating sacrifices electrical performance thereof (insulation breakdown or the like), such reduction has not been desirable.
For the reasons discussed above, it has been lately attempted to use a rectangular wire in which the conductor has a shape similar to a quadrilateral (square or rectangle), as a means for increasing the packing factor. Use of a rectangular wire exhibits a dramatic effect in increasing the packing factor. However, since it is difficult to uniformly apply an insulation coating on a rectangular conductor, and since it is particularly difficult to control the thickness of the insulation coating in an insulated wire having a small cross-sectional area, the use of a rectangular wire does not become common.
The property of an insulation coating required for coil-winding of a motor or a transformer includes a property of keeping electrical insulation unchanged between before and after the coil-working (hereinafter referred to as an electrical insulation keeping property before and after the working). When the coating of an electrical wire is damaged upon the coil-working process, the electrical insulation performance deteriorates, which results in a loss of reliability for products.
Various methods have been conceived as the method of imparting this electrical insulation keeping property before and after the working to the coating of an electrical wire. Examples thereof include a method of reducing surface damage at the time of working into a coil, by imparting a lubricating property to the coating, and thereby lowering the coefficient of friction; and a method of retaining the electrical insulation performance, by improving the adhesiveness between the coating and the electrical conductor, and thereby preventing the coating from being peeled off from the conductor.
As the former method of imparting lubricating property, use has been traditionally employed of a method of applying a lubricant, such as wax, on the surface of an electrical wire; or a method of imparting lubricating property, by adding a lubricant to the insulation coating, and making the lubricant to bleed out to the surface of the electrical wire at the time of producing the electrical wire. There are many examples of the former method. However, since the method of imparting the lubricating property to a coating does not enhance the strength of the coating of the electrical wire itself, the method seems to be effective against the surface damage factors, but there has been in fact limitative on the effect at the time of coil-working.
The above-mentioned method of reducing the coefficient of friction of the surface of the insulation coating, which is a conventionally used means other than the means of imparting a lubricating property to the coating, includes a method of applying wax, oil, a surfactant, a solid lubricant, or the like onto the surface of an insulated wire, as described in Patent Literature 4 or the like. Further, it includes a method of applying a friction reducing agent containing a wax capable of being emulsified in water and a resin capable of being emulsified in water and solidified upon heating, and baking it before use, as described in Patent Literature 5 or the like. Further, it includes a method of enhancing lubrication by adding a fine powder of polyethylene to the insulation coated material itself, as described in Patent Literature 6 or the like. The above methods have been conceived so as to enhance the surface lubricating property of the insulated wire, and to consequently protect the insulation layer from surface damage through surface sliding of an electrical wire.
However, since these methods of adding a fine powder are complicated in the technique of adding the fine powder, and dispersing is difficult, a method of adding such a fine powder in the form of being dispersed in a solvent, into an insulation coated material, is employed in many cases.
These self-lubricating components can have an improvement of the self-lubricating property (coefficient of friction) by the lubricating components, but do not enhance properties such as reciprocating abrasion upon reduction in electrical insulation keeping property before and after the working, and as a result electrical insulation cannot be kept. Furthermore, many types of self-lubricating components, such as polyethylene and poly (tetrafluoroethylene), become separated from the insulation coated material, due to a difference in the specific gravity between the insulation coated material and the self-lubricating components, and therefore a method of using these coated materials has a disadvantage for a practical use.